Quakes often occur in areas where one of the plates that make up the crust of the earth is moving downward, beneath another section of crust, a process called subduction.
But researchers report in Friday's edition of the journal Science that in the Solomon Islands quake there were actually three sections of crust involved, two of them sliding at different rates beneath the third.
The Australia Plate is moving beneath the Pacific Plate at about 5.5 inches-a-year, while the adjacent Solomon Plate is moving about 4 inches-a-year, according to researchers led by geoscientist Kevin P. Furlong of Penn State University.
In addition, the researchers noted, the Australia and Solomon plates are moving in slightly different directions.
The quake started in the boundary between the Australia and Pacific plates, they reported, and then suddenly jumped to the Solomon-Pacific boundary.
"Normally we think earthquakes should stop at the plate boundaries," Furlong said in a statement.
And, he added, when the earthquake moved from one plate to the other, it quickly changed direction, mimicking the different plate motion directions of the plates involved: "I do not know of any other place where we have observed that behavior during an earthquake before, but it most certainly has happened here before."
The combination of motions caused the Pacific plate to bunch up by a couple of yards, the researchers said. And that uplift is probably the reason for the unexpected size of the tsunami waves that followed.
Coral islands are common in that area, Furlong said, and they may be the result of similar previous instances of uplift.
This is also some of the youngest oceanic crust known to be subducting and, according to Furlong, seismologists do not expect young sections of the Earth's crust to be locations of major earthquakes.
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